The present invention relates to a search technology, and more specifically, to a search apparatus, search method and search program using a public search engine.
At present, a variety of public search engines may be provided on the Internet, and can be accessed by anyone. However, when a search is made for confidential information (such as patent information, information on new product development, technical know-how, and personal information), there may be a risk of having the confidential information inferred based on keywords included in the search queries.
In order to resolve this sort of problem, Japanese Patent Application Publication No. Hei 11-259512 (Patent Document 1) discloses a data search system in which a data search apparatus performs a search after concealing or obscuring from the data search apparatus all of or a part of a search condition inputted to the data search apparatus, and then refines a result of the search. In order to conceal or obscure the search condition, this system deletes, from all of inputted search conditions, previously registered search conditions that should be concealed, replaces the inputted search conditions with synonyms or hypernyms thereof, adds an unnecessary search condition to the inputted search conditions, or divides each of the inputted search conditions into several search terms.
However, because this system changes search conditions in the above-described manner, it involves a problem of acquiring only an insufficient amount of necessary information, or acquiring an enormous amount of unnecessary information.
Japanese Patent Application Publication No. 2002-297606 (Patent Document 2) discloses a database access system which allows a user to use an information providing service while treating as confidential a search condition that the user wants to conceal. A client computer transmits to a server computer an encrypted query obtained by encrypting confidential data items that are included in search conditions. While encrypting searched data in the same manner as the confidential data items are encrypted, the server computer performs a data search by encrypted data matching without decrypting the encrypted search conditions.
However, this system uses encryption to merely conceal how search conditions are combined (AND, OR, NOT, etc). Additionally, another problem is that a processing load associated with the encryption and the data matching therefore becomes large.
Other than those mentioned above, there have been proposed the following search methods.
“Private Information Retrieval (PIR)”: There has been a problem that, while a server retains data of N bits, a client acquires 1 bit out of the N bits without informing the server. A simple solution to this problem is that the server should transmit all of the N bits to the client. Since the 1990's, however, there have already been study results indicating that, on the assumption of existence of multiple servers and computational complexity, the problem can be solved without needing transmission of all of the bits. These are mathematical basic theories and have not yet been put to practical use.
“Oblivious Transfer (OT)”: Whereas PIR may allow a client to acquire information other than information wanted by the client, OT allows a client not only to make settings for achieving the search method based on PIR, but also to design a protocol for allowing the client to only acquire data wanted by the client, excluding information other than the wanted data. Additionally, in the case of OT, there is no limit to volumes of communication. OT is the same as Symmetric PIR described in Non-patent Document 1. OT is also a theory, and there have been almost no examples of actual implementation thereof.
“New techniques for private stream searching”: In this method, a query is encrypted on a server side by use of a public key of a user, and an index is encrypted at the same time. Thereby, the user can acquire a search result without making information known to the server. However, because this method needs to be implemented on the server side, users cannot utilize search servers generally used in public.
Japanese Patent Application Publication No. Hei 11-259512, and Japanese Patent Application Publication No. 2002-297606 are examples of the related art.
In addition, Japanese Patent No. 2008-216821 is an example of the related art.
Therefore, there is a need for searching without revealing search words to a search engine.